Metal diborides (MB2) possess outstanding properties for thin film applications in microelectronics and hard coatings: metal diborides such as TiB2, ZrB2 and HfB2 have melting points often exceeding 3000° C., electrical resistivities as low as 15 μΩ-cm, and hardnesses approaching 30 GPa. In addition, they are chemically robust. Some metal borides are attractive as potential replacements for TiN as electrically conductive diffusion barriers in integrated circuits, preventing the interdiffusion of copper and silicon in interconnects (See, S. Jayaraman et al. J. Vac. Sci. Technol., A 2005, 23, 1619-1625, J. W. Sung, J. Appl. Phys. 2002, 91, 3904-3911, U.S. Pat. No. 6,445,023 B1 and 6,872,639 B2). Moreover, magnesium diboride (MgB2), which becomes superconducting below ca. 40 K, is potentially useful for the fabrication of superconductor-based integrated circuits. (See, M. Naito et al. Supercond. Sci. Technol. 2004, 17, R1-R18, U.S. Pat. No. 6,797,341).
Physical vapor deposition (PVD) and chemical vapor deposition (CVD) methods are primarily employed in preparing thin films of metal diborides. In general, the PVD process deposits thin films of the desired composition by generating highly reactive particles from pure bulk targets by evaporation or sputtering followed by transporting them onto the substrate on which the films grow. In CVD, a molecular precursor that contains some or all of the elements in the desired thin film, is vaporized and delivered onto a hot substrate, sometimes in combination with other precursors. Subsequent chemical reactions afford thin films of the desired material on the substrate. CVD methods can produce uniform coatings on high aspect-ratio trenches and holes, which is difficult to accomplish by PVD methods, which tend to be “line of sight” owing to its use of highly reactive particles.
Atomic layer deposition (ALD), which is a variant of the CVD method, is based on self-limiting reactions. A substrate is exposed to a precursor, which reacts to coat the substrate with a monolayer (or less) of material. The excess precursor is then pumped away, and the substrate is exposed to a second precursor, which reacts with the thin layer generated by the first reactant. Then, the excess of this second reactant is removed. By repeating this cycle, or variants in which more than two precursors are employed, the desired thin film can be grown layer by layer. One feature of ALD is that it can provide highly conformal coatings in extremely narrow, deep holes.
The PVD processes that have been performed for the deposition of transition metal diboride thin films include sputtering, co-sputtering, reactive sputtering, pulsed-laser ablation and evaporation. Sputtering from MB2 targets is the most frequently employed method for MB2 deposition among PVD methods; thin films of TiB2, ZrB2, HfB2, VB2, TaB2, and CrB2 have been deposited from each MB2 target.1-5 TiB2 thin films have been prepared by a co-sputtering method that uses separate Ti and B targets (See, J. Pelleg et al. J. Appl. Phys. 2002, 91, 6099-6104; T. Shikama et al. Thin Solid Films 1988, 156, 287-293). Reactive sputtering method using B2H6 gas as a boron source has also been employed to deposit TiB2 thin films. (See, H. O. Blom et al. J. Vac. Sci. Technol., A 1989, 7, 162-165). In all these sputtering methods, the fluxes of metal and boron must be precisely controlled to produce stoichiometric MB2 deposits: different sputter yields, different angular emission profiles, and different gas scattering effects can cause there to be excess metal or excess boron in the films. (See C. Mitterer, J. Solid State Chem., 133, (1997) 279) Failure to produce stoichiometric MB2 films often results in films with less useful properties. Although sputtering itself can be performed at moderate substrate temperatures, high temperature annealing processes are often necessary to obtain films with desired properties. An alternative to sputtering, pulsed-laser ablation, has been employed to produce TiB2 films (See, V. Ferrando, et al. Thin Solid Films 2003, 444, 91-94). One disadvantage of this PVD technique is that a high deposition temperature of 720° C. was necessary. Thin films of TiB2 and ZrB2 have also been prepared by evaporation; the films were, however, nonstoichiometric and required high deposition temperatures of above 1000° C. (See, R. F. Bunshah, Thin Solid Films 1977, 40, 169-182). Although high-quality MB2 films have often been prepared by PVD methods, all these PVD approaches have limited abilities to deposit uniform coatings onto non-flat surfaces.
CVD methods for thin films of transition metal diborides can be divided into two processes in terms of precursor types: 1) processes using metal halide precursors and 2) processes using single sources of metal hydroborate complexes. In the former, metal halides are reduced in the presence of a boron source to metal borides and hydrohalic acids (HX). TiB2 and ZrB2, for example, have been prepared from the reaction of BCl3 and H2 with TiCl4 and ZrCl4, respectively.6,7 Diborane can be used in place of boron halides and H2: the reaction between B2H6 and TiCl4 and TaCl5 produced TiB2 and TaB2 thin films, respectively.8,9 However, CVD processes using halogen-based precursors require high deposition temperatures of 600-1200° C. and often leave behind traces of halogen contaminants both of these features are detrimental for many microelectronic applications. Another disadvantage of this method is that few metal halides are volatile below their decomposition temperatures.
Single source precursors to metal diborides have been described that have the general stoichiometry MBxHy; these bear tetrahydroborate (BH4−) or octahydrotriborate (B3H8−) groups. The precursors Zr(BH4)4, Hf(BH4)4, and Cr(B3H8)2 have afforded the corresponding MB2 thin films at temperatures as low as 150° C.10-12 The single source precursors are free of heteroatoms such as halogens that could contaminate the films, and the deposition temperatures are often lower than 400° C.
For many metal boride phases, it is not possible to use single source CVD precursors to grow films because no precursor of stoichiometry MBxHy exists. Volatile M(BH4)n complexes of d-block transition metals are rare because the BH4− ligand is sterically small and strongly reducing, and in fact only Ti(BH4)3, Zr(BH4)4, and Hf(BH4)4 are known. Because BH4− is sterically small, three or four BH4− ligands are required to saturate the coordination sphere of a transition metal center and form a volatile complex, which accordingly means a +3 or +4 oxidation state for the metal center. Many transition metals, however, are not stable in these oxidation states in the presence of strongly reducing BH4− groups. By employing the sterically more demanding hydroborate ligand, B3H8−, the highly volatile chromium(II) complex of Cr(B3H8)2 has been prepared and have demonstrated its excellence as single source precursor to very high quality CrB2 thin films.12,13 However, although the B3H8− group is sterically larger than the BH4− ligand, so far only chromium has been shown to form a highly volatile transition metal species suitable as a CVD precursor.
The lattice structure of MgB2 is identical with that of the transition metal diborides, but the deposition of MgB2 thin films is complicated by one major challenge: loss of Mg from the MgB2 phase at growth temperatures above ca. 400° C.14 If enough Mg is lost, the MgB2 films become non-superconducting. Several reports of the deposition of MgB2 by PVD methods have appeared. Kang and co-workers have produced MgB2 films by depositing amorphous boron followed by reaction with Mg vapor at 900° C. in sealed tantalum tube.15 Although this method has produced high-quality MgB2 films with a critical temperature Tc of 39 K, the ex-situ high temperature annealing process must be conducted in a sealed tube, which makes this method impractical for producing multilayer thin films on a large scale. Ueda et al. have produced MgB2 thin films with a Tc of ca. 38 K by co-evaporation at 240 to 270° C.16,17 Zeng et al. have grown MgB2 thin films by an in situ hybrid physical-chemical vapor deposition (HP-CVD) method in which B2H6 reacts with Mg vapor generated from Mg chips placed near the substrate.18,19 The main obstacle to employing this latter approach in the fabrication of multilayer devices is the high deposition temperature of ca. 750° C., which will promote undesirable interfacial reactions.
A desirable method for incorporating MgB2 into multilayer devices should produce crystalline, conformal films below 400° C. via an in situ deposition process without the need for a subsequent annealing process at an elevated temperature. To date, only the co-evaporation method comes close to this requirement, but this method cannot afford conformal films on topologically complex substrates. Thus, a need exists for designing and synthesizing volatile magnesium-containing compounds for the CVD of MgB2.